Among raw materials for the preparation of polymer products, the processing of acrylic monomers has undergone rapid development in recent years. Acrylic monomers are used predominantly in the production of fibers, dispersions, raw materials for coatings, raw materials for adhesives, and thermoplastic compositions. In smaller amounts, they serve as starting materials for a variety of chemical syntheses.
Accordingly, there are a large number of processes for preparing such acrylates and/or methacrylates. For the purposes of this invention, "acryloyl" or "methacryloyl" means a radical of the general formula ##STR2##
where R=CH.sub.3 or H.
In addition to conventional processes for the preparation of acrylates and/or methacrylates, which correspond essentially to literature preparation processes for carboxylic esters (Review in J. March, Advanced Organic Chemistry, Wiley, 1992), there are also specifically described processes which are known in connection with the modification of hydroxy-functional siloxanes and/or polyoxyalkylene-modified siloxanes with acrylic and/or methacrylic esters or acrylic and/or methacrylic acid.
In this case, the common processes start from hydroxy-functional precursors and introduce the acryloyl and/or methacryloyl group by esterification or transesterification processes, starting from the corresponding acrylic and/or methacrylic acids or acrylic and/or methacrylic esters. In general, the presence of catalysts is unavoidable for these reactions. U.S. Pat. No. 4,777,265, for example, describes chelate complexes of titanium, zirconium, iron and zinc with 1,3-dicarbonyl compounds as catalysts for such reactions. In many cases, acids are also used to catalyze the esterification reaction, as is the case, for example, in U.S. Pat. No. 5,091,440.
These processes generally take place at temperatures above 80.degree. C., frequently above 100.degree. C., and require additional stabilization of the reaction mixture by means of free-radical scavenging (for example, methylhydroquinone), in order to reliably suppress unwanted polymerization of the acrylates and/or methacrylates at these temperatures. For many fields of application, the catalyst must subsequently be removed, or at least neutralized in order to avoid unwanted side reactions. This requires a complex workup procedure, in which metal oxides, metal hydroxides or corresponding salts of the metals and/or of the acids used as catalysts are formed and then, in general, removed by filtration. Such filtrations of acryloyl- and/or methacryloyl-containing reaction mixtures are complex from a technological and industrial safety standpoint and, consequently, are often lengthy. Because of the high reaction temperature, acryloyl- and/or methacryloyl-functional compounds prepared in this way frequently have an intense coloration (yellow to brownish black). This often prevents the direct use of such acryloyl and/or methacryloyl compounds in applications wherein the coloration requirements of the raw materials are stringent. In this respect mention may be made, for example, of their use as an additive in radiation-curing coatings, especially clearcoats.
Furthermore the direct reaction of alkoxy-, hydroxy- or chlorosiloxanes with hydroxy-functional acrylates and/or methacrylates is described in DE-A-27 47 233. Here again, metal catalysts, elevated temperatures above 120.degree. C., and additional inhibitors are required. Consequently, this process is also hampered by the abovementioned disadvantages in respect of high color numbers, catalyst residues and unwanted polymerization of the acrylate and/or methacrylate groups.
In addition, there are also processes which make use of an at least difunctional linking unit by means in which (See, for example, U.S. Pat. No. 4,218,294, U.S. Pat. No. 4,369,300, U.S. Pat. No. 4,130,708, U.S. Pat. No. 4,369,300, EP-A-0 518 020, WO 86/02652 or DE-A-30 44 301) a hydroxy-functional compound is linked with a hydroxy-functional acrylate (for example, hydroxyethyl acrylate) by reaction with a diisocyanate (for example, isophorone diisocyanate) to form at least two urethane bridges with one another.
Similarly, there are also monomers available commercially which also carry an isocyanate group in the molecule (for example TMI.RTM., meta-isoprenyl-.alpha.,.alpha.-dimethylbenzyl isocyanate; Cyanamid) and are therefore capable of reaction with alcohols. Starting from these toxicologically objectionable parent structures, a urethane group is formed which links the two structural elements to one another. The reaction between isocyanate group and hydroxyl group is generally catalyzed by the addition of catalysts (for example tin compounds or amines). These catalysts remain in the end product. Further linking reactions, by way of oxirane derivatives, for example, are described in DE-A-39 32 460, and J. Appl. Polym. Sci., Vol. 59, 1937-1944.
These processes which introduce additional functional groups into the resultant acryloyl and/or methacryloyl compounds (for example, urethanes, ureas, .beta.-hydroxy esters, esters, etc.) have the practical disadvantage that these additional structural elements exert a limiting effect on possible further modifications to these compounds by means, for example, of secondary reactions. There may also be an undesirable effect on processing parameters, such as the viscosity. In some cases, the structure as well of the organomodified siloxane methacrylates and/or siloxane acrylates prepared in this way is influenced such that desired or unwanted surface-active properties are altered, so as to place an additional barrier in the way of systematic development work.
R. Tor, Enzyme Micro. Technol., 1990, Vol. 12, April, pp. 299-304, describes the enzymatically catalyzed transesterification of acrylic and methacrylic monomer esters for the preparation of hydroxy- and dihydroxyalkyl acrylates and methacrylates without the formation of di- or triacrylates and -methacrylates. 2-hydroxyethyl, 2-hydroxypropyl and 1,2-dihydroxypropyl esters of acrylic acid and methacrylic acid are investigated in the R. Tor disclosure.